Band-pass selector system



Oct. l, 1940 L. F. CURTIS BAND-PASS SELECTOR SYSTEM Filed June 30, 1959 op-assai osuodslgnnop Mmmm @www INVENTOR w-f. cum-is.

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AT'H'URNEY Patented Oct. l, 1940 BAND-PASS SELECTOR SYSTEM Leslie F. Curtis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Dela- Ware Application June 30, 1939, Serial No. 282,143

12 Claims.

This invention relates to band-pass selector systems and, more particularly, to such systems in which the width of the frequency band translated by the system is adjustable. `While bandpass selector systems constructed in accordance with the present invention are of general utility, they are particularly suitable for use in the intertermediate-frequency channel of modulated-carrier signal receivers of the superheterodyne type for controlling the selectivity and fidelity of response of the receiver.

Band-pass selectorsystems of conventional design usually comprise a pair of resonant circuits tuned to the same `or different frequencies and suitably coupled, inductively, capacitively, or by a combination of these individual couplings. The desired response of this type-of selector system is substantially uniform over a band of frequencies in the vicinity ofthe mean resonant frequency of the selector, while signal components of all other frequencies are sharplydiscriminated against and attenuated to a substantial degree by the selector. In general, the width of the frequency band passed by such a selecto-r system may be varied by adjusting the coupling between the two circuits, by adjusting the resonant frequencies of the two circuits relative to each other, or by a combination of these two adjustments. Selector systems of the type described generally utilize coupling systems which are nondirective in nature; that is, either circuit may be the input circuit and the other the output circuit without substantially affecting the characteristics of the system. Such a system is to be contrasted with that type of selector utilizing a `vacuum-tube coupling in one or both directions between the input and output circuits wherein one or both of the couplings are primarily unidirective.

Band-pass selector systems wherein the nondirective form of coupling is utilized are, in general, open to the criticism that only mechanical, or relatively complicated nonmechanical, arrangement are known for adjusting the width of the frequency band to be translated. Band-pass selector systems employing unidirective couplings in both the forward and backward directions between the tuned circuits, however, require a separate vacuum tube, or the equivalent, in each of the forward and backward coupling paths. All of the above-mentioned band-.pass selectors, moreover, require two resonant circuits.

It is an object of the present invention, therefore, to provide an improved band-pass selector system of simple arrangement in which one or more of the above-mentioned disadvantages of prior artsystems are eliminated.

It is another object 'of the invention to provide an improved band-pass selector system requiring only one tuned circuit in the signal- 5 translating channel with which it is associated.

In accordance with the present invention, a modulated-carrier signal-translating stage comprises -a` tuned circuit including parameters of resistance, capacitance, and inductance, and nor- 10 mally resonant in the vicinity of the frequency ofr the carrier` wave to be translated. There is provided meansfor varying one of the parameters of the tuned circuit at a frequency twice that of the carrier waveto be translated by the l5 stage, whereby the selectivity of the stage depends on both the timing of the variations with respect tothe carrier frequency of the translated signal and the amplitudeor range of the variations. There is also provided means for ad- 20 justing the relation between the amplitude of the variations and the phase angle between the variations and the signal-carrier wave to impart predetermined adjustable selectivity characteristics to the stage. 25

In accordance with a preferred embodiment of the invention, the capacitance of the tuned circuit is varied sinusoidally at twice the frequency of the carrier wave while the actual conductance of the tuned circuit is'maintained substantially 30 constant. In this preferred embodiment of the invention an oscillator, synchronized by means of the signal-carrier wave to be translated by the stage, is utilized to generate a frequency at twice that of the signal-carrier wave. These oscillationsare utilized to Vary thetransconductance of a control vacuum tube which, in turn, is utilized as an adjustable capacitance across the tuned circuit, for example, in accordance with the well-known Miller effect. Also in accordance with the preferred embodiment, the timing or phase of the generated oscillations may be adjusted with respect to that of the signal-carrier wave, the bias applied to the control tube may be adjusted to control the maximum amplitude of the capacitance Variations of the tuned circuit, and these two adjustments may be relatively adl justed to impart predetermined adjustable selectivity characteristics to the` stage.

For a better understandingv of the invention, togetherwith-other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. y

Fig. 1 of the drawing is a circuit diagram, partly schematic, of a modulated-carrier signal receiver of the superheterodyne type employing a band-pass selector constructed in accordance with the invention; Fig. 2 is an equivalent circuit diagram of a portion of the circuit of Fig. 1 utilized to explain the general theory of the invention; while Figs. 3 and 4 are graphs illustrating certain of the operating characteristics of the circuit of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, there is shown a circuit diagram, partly schematic, of the improved band-pass selector system of the invention embodied in a modulated-carrier signal receiver of the superheterodyne type for controlling the selectivity of the intermediate-frequency channel of the receiver. Briefly described, the receiver comprises a radio-frequency amplier I0 having input terminals connected to an antenna ground circuit II, I2, and'output terminals coupled to a frequency changer or oscillator-modulator I3. lConnected in cascade with the oscillator-modulator unit I3, in the order named, are an intermediate-frequency amplifier I4 including a vacuum-tube amplifier I5, variable band-pass selector I6, an intermediate-frequency amplifier I 'I including a vacuum-tube amplifier I 8, a detector and automatic amplification control or A. V. C. unit I 9, an audio-frequency amplifier 20, and a sound reproducer 2|. The bias potential derived from the automatic amplification control source I9 may be applied to one or more stages of radio-frequency amplifier I0, the modulator included in unit I3, and one or both of the stages of intermediate-frequency amplifica- -tion I4 and I'I.

Considering first the operation of the receiver as a whole without regard to the details of the band-pass selector I6 of the invention, per se, a desired received signal is selected and amplified -in radio-frequency amplifier I0 and is converted to a modulated intermediate-frequency signal by oscillator-modulator I3. The signal as thus converted is further amplified in intermediate-frequency amplifier I4, is selected by variable bandpass selector I6, is further amplified in intermediate-frequency amplifier I'I, and detected by detector I 9, thereby producing the audio-frequency modulation components which are, in turn, amplied by audio-frequency amplifier 20 and reproduced by the sound reproducer 2|. The amplification of the received signal is subject to automatic control by the control-bias potential derived from source I 9 to maintain the amplitude of the signal input toy detector I9 relatively constant for a wide variation of received signal amplitudes, in a manner well understood in the art.

Referring now more particularly to the portion of the system of Fig. 1 comprising the present invention, there is provided a band-pass selector I6 including a single tuned circuit 25, 26 in the signal-translating channel of the receiver 'and utilized as a coupling circuit between vacuum tubes I5 and I8. It will be understood that only the portion of the circuits of tubes I5 and I8 necessary to describe the present invention are shown in the drawing and that these tubes are provided with additional circuits in a manner well understood in the art toy effect operation thereof as conventional vacuum-tube amplifiers. In order to vary the selectivity of tuned circuit 25, 26, the eifective capacitance thereof is pulsate-d at a frequency approximately twice that of the intermediate-frequency carrier wave. For

this purpose, there is provided a vacuum tube ,21 having input electrodes coupled across tuned circuit 25, 26 and having its grid and anode reactively coupled by a condenser 45, thereby being effective to reflect capacitance into tuned circuit 25, 26 in accordance with the well-known Miller effect.

In order to vary the transconductance of tube 2'I at a frequency twice that of the intermediatefrequency carrier wave, thus to pulsate the capacitance across tuned circuit 25, 26 at the double frequency, there is provided a vacuum-tube oscillator 28 tuned to generate oscillations at this double frequency by means of a frequency-determining circuit 29, 30 coupled to its input electrodes and a feed-back circuit including an inductance 3| in its output circuit and inductively coupled to inductance 29. In order to synchronize the oscillator 28 with the carrier wave of the intermediate-frequency signal of the receiver, a voltage is coupled to the oscillator from the intermediate-frequency channel of the receiver through a vacuum-tube limiter 32. Tube 32 operates as a conventional limiter and is provided in order to eliminate the effects of the amplitudemodulation components or amplitude variations of the intermediate-frequency signal. Tube 32 comprises a tuned output circuit including a condenser 33 and inductance 34 inductively coupled to feed-back inductance 3| of the oscillator circuit.

A phase-shifting network including a variable condenser 35 and resistor 36 is provided for varying the phase of the synchronizing signal applied to oscillator 28, thereby to contro-l the timing or phase of the lgenerated oscillations. The output oscillations of oscillator 28 are coupled into the cathode circuit of the control tube 21 by means of an inductance 38 inductively coupled to inductance 29 of the oscillator circuit. A source of variable bias including a resistor 39 connected to an adjustable tap on resistor 4I is provided for vacuum tube 2'! in order that the magnitude of the capacitance variations reflected into tuned circuit 25, 26 may be varied. Operating potentials for tubes 21, 2B, and 32 may be supplied from suitable sources indicated as +B and -l-Sc, the anode circuit of control tube 21 including a load resistor 40.

Reference is now made to Fig. 2 for an explanation of the eiect of varying the parameters of a tuned circuit at twice the carrier frequency of a modulated-carrier signal applied thereto. In Fig. 2 there is shown a tuned circuit comprising parallel-connected inductance L and capacitance C effectively shunted by a resistor having a conductance g. Conductance g and capacitance C are indicated fas being variable for the reason that the circuit is to be analyzed under the condition of periodic pulsation of each of these parameters at twice the carrier frequency of the system. The tuned circuit L, C is thus the equivalent of the effective inductance and capacitance associated with tuned circuit 25, 26 of Fig. 1 and the current in is thus effectively the equivalent of the output current supplied by tube I5. It is assumed for the purposes of analysis that:

g=yoi1+2q sin (Zot-tw] (2') C=Co[1|-27c sin (Zot-tip] (3) Where:

q=half the percentage pulsation of conductance y; i

and the terms in Equation 1 are those corresponding to the conventional expression for a modulated-carrier current having symmetrical upper and lower sdebands.

Under these assumptions the differential equation applicable to the circuit of Fig. 2 is:

ge-l-C' de/dt+fedt/L=io (4) where: t ztime; e--the instantaneous voltage across the circuit;

`:v -:the angular frequency of the input-carrier Wave; and i a=the angular frequency of the modulation of the input-signal wave The steady-state solution of this equation contains terms at angular frequencies w, w+a, w-a, w+2a, rfi-2a, etc. The magnitudes of all the terms except the rst three of the above are small and may be neglected. i

e may then be assumed to be:

:A cos wt-i-B sin wt -i-X cos (w-aJt-l-Y sin (w-aJt (5) where A, B, F, H, X, and Y are the coefficients associated-with the various terms of the equation. When the inductance L and the mean capacitance Co are tuned to the frequency of the inputcarrier wave, the value of w may be designated by an, that is The steady-state solution of differential Equation 4 then provides the following:

Eszthe R. M. S. sideband Voltage across L, C; a :Lt/wo; ltl'ld p :the power factor of the original circuit.

qp cos qb-i-k sin 41:0 (8) This is most easily satisfied by making =i1r/2 and 111:0 or 1r. If lc=0 or q=0, respectively, these are the only angles which give symmetry. Therefore, if q==0, cos w=i1 for symmetrical expansion, or if lc=0, sin =i 1.

The response characteristic of a circuit having regenerative feedback and tuned to the input signal is well known and may be shown to be represented by the equation:

where pn is the absolute value of the effective power factor introduced by a negative conductance across the tuned circuit and the other terms are as in Equation '1. Attention is called to the fact that the expression of Equation 9 is of exactly the same form as that of Equation '1 above.

In accordance with the circuit arrangement shown in Fig. 1 only the parameter of capacitance of the tuned circuit 25, 26 is varied, which means that the factor q in the above equations is zero. This embodiment is preferred since calculations indicate that detuning produced by fundamental frequency effects in tube 21 may be compensated by a readjustment of the value of either of the elements of tuned circuit 25, 2B. The harmonic effects in tube 21 produce beatfrequency currents in its output circuit due to the difference in frequency of the applied Voltages. The effective sideband Voltage output of the circuit of Fig. 1 for a signal input comprising symmetrical upper and lower sidebands may be expressed by the equation:

i 1/cam/ p+1-tA2dLa2E eos @una where The term RA2C1wn2E cos 0 has the dimensions of a power factor by which the original power factor of tuned circuit 25, 2B is increased by the doublefrequency pulsations. Obviously, the term may be made negative and the circuit regenerative by making 0:1r.

In summary, therefore, it is seen that the arrangement of Fig. 1 comprises means for varying the reactance of tuned. circuit 25, 26 at twice the frequency of the intermediate-frequency carrier wave; that the capacitance of the tuned circuit 25, is pulsated by means of a tube 21 in accordance with the Miller effect; that oscillator 28 generates oscillations at twice the carrier frequency Which are used to vary the transconductance of the Miller tube 21; that the phase of these generated oscillations can be adjusted by adjustment of condenser 35, thereby to adjust the phase of the synchronizing signals applied to oscillator 28; that the limiter 32 is effective to remove the amplitude-modulation components from the signal utilized to synchronize oscillator 28; and that the amplitude of the capacitance pulsations of circuit 25, 26 may be adjusted by adjusting the tap on resistor 4| in the manner described above.

Reference is made to Fig. 3 for a description ill) of the eiect of pulsating only one of the param'-A eters of tuned circuit 25, 26. In Fig. 3, curve A represents the normal response characteristic of tuned circuit 25, 26. If the capacitance of circuit 25, 26 is varied at twice the carrier frequency of the system and the phase of these capacitance pulsations is zero with respect to that of the intermediate-frequency carrier wave; that is, if the carrier frequency goes through zero amplitude in the positive direction at the same time that the capacitance pulsation is changing through zero in the positive direction, tuned circuit 25, 26 has a response characteristic as illustrated by curve B. It is assumed that, in his case as in the case of curve A, there is no conductance pulsation of tuned circuit 25, 26. The corresponding characteristic of the system of the phase angle is equal to 1r, all other conditions being unchanged is shown by curve C.

. On the other hand, if the capacitance of the circuit is maintained constant and the conductance is pulsated at a frequency equal to twice that of the carrier frequency and with a phase of conductance pulsation equal t0 +1r/2 with respect to the signal-carrier wave, the tuned circuit 25, 26 has a response characteristic as shown by curve C of the drawing. The corresponding characteristic for a phase angle of 1r/2 is that of curve B, all other conditions being unchanged. As explained above, a pulsation of capacitance at phase angles of other than zero or fr results in an unsymmetrical response characteristic. The response of the system for capacitance pulsation at a phase angle of 1r/4 is shown by curve D while the corresponding characteristic for the phase angle of +1r/4 is shown in curve E.

Characteristic curves showing the response of the system when the capacitance of tuned circuit 25, 26 is pulsated with varying amplitudes at twice the carrier frequency of the system and with phase angles of zero and 1r with respect to the signal-carrier wave are shown in Fig. 4. Thus, curve A represents the normal resonant characteristic of tuned circuit 25, 26 and corresponds to curve A of Fig. 3. If the amplitude of the capacitance pulsations is varied under the conditions outlined above and with a phase angle of zero, the response characteristic of tuned circuit 25, 26 becomes increasingly more broad as shown by curves A, F, and B, curve B thus cor-l responding to curve B of Fig. 3. Such an increase in amplitude of capacitance pulsations is eiected by adjustments of the tap on resistor di. Corresponding characteristics for a phase angle of 1r, all other conditions remaining unchanged, are shown by curves A, G, and C.

From the foregoiing, it is apparent that the tap on resistor 4I and the value of capacitance -35 may be relatively adjusted to procure predetermined selectivity characteristics for the intermediate-frequency channel of the receiver shown. It will be understood that, while in the embodiment illustrated in Fig. 1 the capacitance of tuned circuit 25, 26 is pulsated, it is equally practicable to pulsate the inductance thereof, or a reactance of either kind in the circuit 25, 26 may be pulsated simultaneously with the conductance thereof in order to provide a readily and flexibly adjustable band-pass selector.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover allsuch changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed-ts: 1. A'modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capacitance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means for varying oneof said parameters at a frequency of twice that of said carrier frequency, Wherebythe `selectivity of said stage depends on both the phase angle of said variations with respect to the carrier wave of said translated signal and the amplitude of said variations, and means for providing a predetermined relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

2. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capacitance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means for varying one of said parameters at a frequency twice that of said carrier frequency, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to the carrier wave of said translated signal and the amplitude of said Variations, and means for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

3. A modulated-carrier signal-translating stage comprising, a tuned circuit including reactance of one kind and normally resonant in the vicinity of the frequency o f the carrier wave to be translated, means for varying said reactance to vary the resonant frequency of said tuned circuit at a frequency twice thatof said carrier frequency, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to said translated carrier signal and the amplitude of said variations, and means for adjusting the relation between the Value of the amplitude of said frequency variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

4. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capictance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means for varying sinusoidally one of said parameters at a frequency twice that of said carrier frequency, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to said translated carrier signal and the amplitude of said variations, and means for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

5. A modulated-carrier signal-translating stage comprising, a tuned circuit including capacitance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means for varying said capacitance to vary the resonant frequency of said tuned circuit at a frequency twice that of said carrier frequency, whereby. ,the ...selectivity -Qtaid YStage depends onboth thephase angle of; said variations with respect to said translated carrier signal and the vamplitude of said variations, and means for adjusting the relation foflthe value of the amplitude of said Variations andthe value of the phase angle between said variationsand said carrier wave to provide predetermined selectivity characteristics for said stage.

6. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capacitance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means comprising a vacuum tube simulating an adjustable impedance coupled to said tuned circuit, means for controlling said vacuum tube at a frequency twice that of said` carrier frequency to vary one of said parameters, whereby the` selectivity of said stage depends on both the phase angle of said variations with respect to said translated carrier signal and the amplitude of said variations, and means for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

7. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, inductance and capacitance and normally resonant inthe vicinity of the frequency of the carrier wave to be translated, an oscillator tuned to generate oscillations at twice the resonant frequency `of said tuned circuit, means comprising said oscillator for varying one of said parameters at the frequency of said oscillations, whereby the selectivity of said stage depends on both the phase angle of Said variations with respect to said translated carrier signal and the amplitude of said variations, and means for adjustingthe relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

8.` A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capacitance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, an oscillator for generating oscillations at twice the resonant frequency of said tuned circuit, phase control means for Ysaid oscillator, means responsive to said oscillations for varying one of said parameters at a frequency twice that of said carrier frequency, whereby the selectivity of said stage depends on. both the phase angle of said variations with respect to said translated carrier signal and the amplitude of said variations,

and means comprising said phase control for said oscillator for adjusting the relation between the value of the amplitude of said frequency variations and the value of the phase angle between said frequency variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

9. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, inductance and capacitance and normally resonant in the vicinity of the frequency of the carrier wave tobe translated, an oscillator for generating oscillations at twice the resonant frequency of said tuned circuit, means for deriving a synchronizing signal for said oscillator from the signal `to be .translated by said stage, means for adjusting the phase of said derived signal, means comprising vsaid oscillations for varying one of said parameters ata frequency twice that of said carrier frequency, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to said translated signal and the amplitude of said variations, and means comprising said phase control for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

10. A modulated-carrier `signal-translating stage comprising, a tuned circuit including parameters of resistance, inductance and capacitance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means for varying one of said parameters at a frequency twice that of said carrier frequency, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to said translated carrier signal and the amplitude of said variations, means for adjusting the amplitude of said variations, and means comprising said last-mentioned means for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

ll. A modulated-carrier signal-translating stage comprising, a tuned circuit including parameters of resistance, capacitance and inductance and normally resonant in the vicinity of the frequency of the carrier wave to be translated, means comprising a vacuum tube simulating an adjustable impedance coupled to said tuned circuit, means for controlling said vacuum tube at a frequency twice that of said carrier frequency to vary onev of said parameters, whereby the selectivity of said stage depends on both the phase angle of said variations with respect to` said translated carrier signal and the amplitude of said variations, bias-control means for said vacuum tube to adjust the amplitude of said variations, and means comprising said bias control for adjusting the relation between the value of the amplitude of said variations and the value of the phase angle between said variations and said carrier wave to provide predetermined selectivity characteristics for said stage.

12. In a modulated-carrier wave-signal receiver of the superheterodyne type, a signaltranslating stage comprising, a tuned circuit in the intermediate-frequency channel of said receiver, said tuned circuit being normally resonant in the Vicinity of the frequency of the intermediate-frequency signal-carrier wave to be translated bysaid channel and including parameters of resistance, inductance and capacitance, an oscillator tuned to generate oscillations at twice the resonant frequency of said tuned circuit, means for deriving a voltage from the signal in- 'put to said stage for synchronizing said oscillathe selectivity of said stage depends on both the phase of said variations With respect to said translated signal-carrier Wave and the amplitude of said variations, an adjustable bias vfor said vacuum tube to adjust the amplitude of said variations, and means comprising saidphase control and saidv adjustable bias for adjusting the relation between the Value of the amplitude of said variations and the value of the phase angle between said variations and said carrier Wave to provide predetermined selectivity characteristics for said stage.v

LESLIE F. CURTIS.

read

Certificate of Correction Patent No. 2,216,160. October 1, 1940. LESLIE F. CURTIS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line 43-44, for the word arrangement read arrangements; page 2, second column, line 72, equation 3, after 2wt-l-xlf and before the bracket insert a parenthesis; page 3, first column, line 46-47, equation 6, for

. page 4, first column, line 14, for his read this; line 57, for foregoiing read foregoing; and second column, line 54, claim 4, for capictance read capacitance; and that the said Letters Patent should be read With these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 3rd day of December, A. D. 1940.

[SEAL] HENRY VAN ARSDALE,

Acting Commissioner of Patents. 

